The IEEE 802.11 MAC standard family defines a way wireless local area networks (WLANs) can work at the physical and medium access control (MAC) level. Typically, the 802.11 MAC (Medium Access Control) operating mode implements the well-known Distributed Coordination Function (DCF) which relies on a contention-based mechanism based on the so-called “Carrier Sense Multiple Access with Collision Avoidance” (CSMA/CA) technique.
The 802.11 medium access protocol standard or operating mode is mainly directed to the management of communication nodes waiting for the medium to become idle so as to try to access to the medium.
The network operating mode defined by the IEEE 802.11ac standard provides very high throughput (VHT) by, among other means, moving from the 2.4 GHz band which is deemed to be highly susceptible to interference to the 5 GHz band, thereby allowing for wider frequency contiguous channels of 80 MHz, two of which may optionally be combined to get a 160 MHz composite channel as operating band of the wireless network.
The 802.11ac standard also tweaks control frames such as the Request-To-Send (RTS) and Clear-To-Send (CTS) frames to allow for composite channels of varying and predefined bandwidths of 20, 40 or 80 MHz, the composite channels being made of one or more channels that are contiguous within the operating band. The 160 MHz composite channel is possible by the combination of two 80 MHz composite channels within the 160 MHz operating band. The control frames specify the channel width (bandwidth) for the targeted composite channel.
A composite channel therefore consists of a primary channel on which a given node performs EDCA backoff procedure to access the medium, and of at least one secondary channel, of for example 20 MHz each. The primary channel is used by the communication nodes to sense whether or not the channel is idle, and the primary channel can be extended using the secondary channel or channels to form a composite channel.
Given a tree breakdown of the operating band into elementary 20 MHz channels, some secondary channels are named tertiary or quaternary channels.
In 802.11ac, all transmissions, and thus all possible composite channels, include a primary channel. This is because the nodes perform full Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) and Network Allocation Vector (NAV) tracking on the primary channel only. The other channels are assigned as secondary channels, on which the nodes have only capability of CCA (clear channel assessment), i.e. detection of an idle or busy state/status of said secondary channel.
An issue with the use of composite channels as defined in the 802.11n or 802.11ac is that the 802.11n and 802.11ac-compliant nodes (i.e. HT nodes standing for High Throughput nodes) and the other legacy nodes (i.e. non-HT nodes compliant only with for instance 802.11a/b/g) have to co-exist within the same wireless network and thus have to share the 20 MHz channels.
To cope with this issue, the 802.11n and 802.11ac standards provide the ability to duplicate control frames (e.g. RTS/CTS or CTS-to-Self) in an 802.11a legacy format (called as “non-HT”) to establish a protection of the requested TXOP over the whole composite channel.
This is for any legacy 802.11a node that uses any of the 20 MHz channel involved in the composite channel to be aware of on-going communications on the 20 MHz channel used. As a result, the legacy node is prevented from initiating a new transmission until the end of the current composite channel TXOP granted to an 802.11n/ac node.
As originally proposed by 802.11n, a duplication of conventional 802.11a or “non-HT” transmission is provided to allow the two identical 20 MHz non-HT control frames to be sent simultaneously on both the primary and secondary channels forming the targeted or requested composite channel.
This approach has been widened for 802.11ac to allow duplication over the channels forming an 80 MHz or 160 MHz composite channel. In the remainder of the present document, the “duplicated non-HT frame” or “duplicated non-HT control frame” or “duplicated control frame” means that the node device duplicates the conventional or “non-HT” transmission of a given control frame over each secondary 20 MHz channel of the 40/80/160 MHz operating band.
In practice, to request a composite channel (equal to or greater than 40 MHz) for a new TXOP, an 802.11n/ac node does an EDCA backoff procedure in the primary 20 MHz channel. In parallel, it performs a channel sensing mechanism, such as a Clear-Channel-Assessment (CCA) signal detection, on the secondary channels to detect the secondary channel or channels that are idle (channel state/status is “idle”) during a PIFS interval before the start of the new TXOP (i.e. before the backoff counter expires).
More recently, Institute of Electrical and Electronics Engineers (IEEE) officially approved the 802.11ax task group, as the successor of 802.11ac. The primary goal of the 802.11ax task group consists in seeking for an improvement in data speed to wireless communicating devices used in dense deployment scenarios.
In such 802.11ax research context, the need becomes apparent for enhancing the efficiency and usage of the wireless channels. Typically, the user demands are primarily related to the delivery of high-definition audio-visual real-time and interactive content in dense WLAN scenarios. It is well-known that the performance of the CSMA/CA protocol used in the IEEE 802.11 standard deteriorates rapidly as the number of stations and the amount of traffic increase.
There is thus a need to improve the communication medium access for multiple users/stations for performing transmissions, and in particular concurrent transmissions.